I played in a game once where the Nine Hells was similar to the Divine Comedy and the DM was very knowledgeable about the middle ages, the Goetia, all that stuff. A lot of that is pretty esoteric these days though.
The secondary DM for my group uses the following homebrew concept for his lower planes cosmology:
There is only the Abyss.
The top six layers are governed by the five chromatic dragon lords (homebrew Demi-gods), with the sixth layer being governed by Tiamat herself.
The nine middle layers are governed by the devils, with the ninth (15th layer overall) being the domain of Asmodeus.
Then come the 66 layers of the Demons, with the 65th (80th overall) being the layer of Orcus, and the 66th (81st overall) being the realm of Demogorgon.
Below these are the 99 layers of the qlippoth (taken from pathfinder).
What resides below the qlippoth is anyone's guess.
In our game the Hells, the Abyss, etc are all the same place. The difference is that demons were already there. A demon's appearance is rather random, very much like the AD&D chart in Dragon 13. Devils are those who betrayed their superiors, who are punished with exile and in appearance are branded, marked and marred. Malkizid is one example.
I have Tiamat back in charge of Avernus, with schemes to take all of the hells. After conquering Faerun and then all of Toril in RoT she stormed back to the hells, smashed Zariel and reinstated Bel as her general.